Since it is difficult to accurately predict the supply and demand of electric power, an energy storage device is required to effectively manage energy.
As for the supply of electric power, it is most economical to maintain a constant optimal output in a system in which heat is used to generate electrical power, such as thermal power generation and nuclear power generation. In contrast, in a system in which the forces of nature is used to generate electric power, such as hydroelectric power generation, wind power generation, and photovoltaic power generation, an output is varied by the natural environment such as seasons, whereas the demand of electric power is varied by various factors such as a power transmission accident, an operation of large-scale factory, a change of the natural environment such as a change of days and nights and a change of season.
Due to characteristics of power energy that is simultaneously supplied and consumed, it costs a large expenditure of money to cause an electric power facility to meet a maximum demand amount, and facilities and human resources necessary to control are required to cause the output to be adjusted depending on the demand.
A lifespan of the power generation facility is reduced in the output variation procedure described above, and minor mismatch between a demand amount and a supply amount causes various problems such as a decrease in quality of electric power.
In order to solve such problems, various electric power storage technologies have been developed.
Among conventional energy storage technologies, a pumped power generation technology, a compressed air storage gas turbine technology, a battery energy storage technology, a superconducting magnetic energy storage technology, and a flywheel energy storage technology have been currently used or developed.
Among systems for implementing such technologies, the flywheel energy storage system is a device that rotates a motor using dump power, stores inertial energy of an attached rotational body, and converts the inertial energy into electric energy to use when necessary.
The flywheel energy storage system has advantageous in that energy storage efficiency is high, an instantaneous charge or discharge is possible, and a lifespan of energy is increased, a decrease in performance does not occur in a low temperature, compared to an existing mechanical energy storage device and a chemical energy storage device.
Due to such features, the system has been used in various fields to the military sector from the private sector such as an auxiliary power unit of an electric vehicle, an uninterruptible power supply, a pulse power generator, and an artificial satellite.
The flywheel energy storage system includes a flywheel rotor for storing inertial energy generated when rotating, a motor for driving the flywheel rotor, a generator for generating electrical power, a controller for controlling an input and output of power, a magnetic bearing serving as a peripheral device, and a housing.
More particularly, the flywheel includes a rotor, a rotational shaft and a hub for fixing the rotor and the rotational shaft.
When the flywheel rotates, since the rotational shaft does not easily expand in a radial direction and the rotor further expands in the radial direction, the hub needs to connect them. Thus, the hub needs to easily expand when the flywheel rotates to connect the rotational shaft and the rotor, and also need to be deformed to transfer torque of the rotational shaft to the rotor.
Further, in order to increase a resonance frequency of a rotation system, that is, the flywheel, than an operation speed, the stiffness needs to increase. Rotational motion energy capable of being stored in the flywheel energy storage system is expressed by the following equation.E=(1/2)Iω2 
As expressed by the above equation, energy stored in the flywheel is proportional to linearly a polar moment of inertia and the square of the rotation speed.
Accordingly, it can be seen that the rotation speed other than a size of the flywheel is highly efficient to increase the stored energy.
However, since an ordinary metal conventionally used for a material of the flywheel has a low tensile stress, it is difficult to rotate at a high speed, so that it adversely affects the high-speed flywheel energy storage system.
Recently, a composite material of high strength has been rapidly developed, so that the rotation speed of the flywheel is increased up to a high speed of 1100 m/sec. Further, energy density per a unit weight and a unit volume of the flywheel is remarkably increased, so that it is possible to develop an energy storage system of a large output.
Especially, since the composite material is fatally damaged by low-strength tensile stress in a radial direction among internal stresses, multiple layers of rings made from the composite material are combined, and an inner ring of the composite material expands toward an outer ring of the composite material, so that the stress may not occur.
Unfortunately, in order to connect the rotor with the rotational shaft, the hub for connecting the rotor and the rotational shaft needs to easily expand in the radial direction, and thus it requires that the hub is designed to easily expand in the radial direction. That is, when the flywheel rotates at a high speed, the hub may be separated from the rotor, so that it is necessary to consider a concern about a firm bond between the hub and the rotor.
In general, the rotor and the hub of flywheel are required to be designed so as to set the number of resonance rotations for avoiding the number of operation rotations and to reduce the internal stress generated during the high-speed rotation.
To achieve this, there have been proposed new hubs for fixing the rotor and the rotational shaft by designers.
FIG. 1 is a cutaway perspective view of a conventional flywheel using a split dome type hub, and FIG. 2 is a cutaway perspective view illustrating the hub of FIG. 1 (see Korea Patent Laid-Open Publication No. 10-2006-0066765).
In the conventional flywheel shown in FIGS. 1 and 2, a plurality of slits 22 is formed in a hub 50 in contact with an inner surface of a rotor 10 in a shaft direction of a rotational shaft 30, and when the flywheel is rotated at a high speed, divided portions, that is, the slits 22 expand in a radial direction by a centrifugal force to allow the hub to apply a compressive force to the inner surface of the rotor, so that it is possible to reduce tensile stress generated in the radial direction of the rotor 10 during the high-speed rotation, and it is possible to prevent the rotor 10 and the hub 50 from being separated from each other. Further, the hub 50 are fixed to the rotational shaft 30 in two or more portions, and thus it is possible to avoid resonance by allowing a resonance frequency of the flywheel t be higher than the operation speed thereof.
Disadvantageously, in the conventional split dome type hub, split-wing portions, that is, portions divided by the slits are moved in the radial direction by the centrifugal force during the high-speed rotation, so that stress concentration may occur at ends of the wing portions, that is, both ends of the slits. As a result, such a stress concentration poses a problem that the hub is damaged. Further, it may be difficult to easily manufacture the hub due to a complicated shape, and it may cost way too much to manufacture.
As explained above, in order to increase the stored energy, the flywheel needs to rotate at a high speed, and the composite material is appropriate to reduce the tensile stress and increase the strength. For this reason, the rotor formed by winding the composite material in multiple layers is used.
However, the rotor formed in the multi-layered composite material has a weakness that the rotor has high strength in a circumference direction but has low strength in the radial direction. That is, during the high-speed rotation, the wound composite material may be tore in the radial direction. Thus, a gap may be generated between the hub and the rotor, or the hub may be separated and deviated from the rotor.
In order to prevent the rotor and the hub from being separated, the hub also needs to expand in the radial direction. In this way, it is possible to prevent the rotor and the hub from being separated from each other.
The hub needs to expand in not only the radial direction. Besides, the hub needs to have enough strength so as not to be damaged during the high-speed rotation and the hub also needs to have a structure or a shape capable of increasing the resonance frequency of the flywheel.